Project Summary: Bladder cancer is the fifth most common cancer in the U.S. Up to 80% of patients initially present with non-muscle invasive bladder cancer (NMIBC) confined to the mucosa of the bladder. These patients are treated by tumor resection followed by intravesical chemotherapy or Bacillus Calmette-Gurin (BCG) therapy. Despite treatment, up to 60% of patients will experience tumor recurrence and, of these, about 50% will undergo radical cystectomy in an attempt to control disease progression and 30% of these patients will progress and succumb to their disease. Due to its high recurrence rate, NMIBC is the costliest of all cancers to treat. Since the FDA has not approved a drug for NMIBC in the last 20 years, novel therapies are desperately needed to prevent NMIBC disease progression and allow bladder preservation. T cells and natural killer (NK) cells have been implicated as critical mediators of the antitumor immune response using BCG therapy. Interleukin-15 (IL-15), considered by the NCI as the most promising immunotherapeutic that could potentially cure cancer, is a crucial factor for effector NK cell and CD8+ memory T cell stimulation and exhibits potent efficacy against tumors in various animal models. To advance IL-15-based therapies into clinical development, we have isolated a novel proprietary IL-15 superagonist mutant and associated it with an IL-15 receptor ??Fc fusion protein to generate a complex referred to as ALT-803. In numerous preclinical models, ALT-803 was shown to have improved pharmacokinetic, biodistribution and immunostimulatory properties and greater antitumor activity than IL-15 alone. We postulate that intravesical treatment of ALT-803 in combination with BCG will induce durable cell-mediated immune responses providing antitumor efficacy in patients with NMIBC. In clinically relevant rodent NMIBC tumor models, we found that intravesical ALT-803 + BCG therapy provided significantly better immune responses and antitumor activity than BCG monotherapy. Based on these results, a multicenter Phase 1/2 clinical trial of intravesical ALT-803 + BCG in NMIBC patients was initiated under the SBIR Phase II project and the dose escalation portion has now been successfully completed. The recommended Phase 2 dose of ALT-803 was established based on a safety profile comparable to BCG monotherapy. High-response rate with impressive durability (i.e., no disease recurrence at up to 18 months) and bladder localized immune cell activation were observed in all treated patients. These findings strongly support advancing to the randomized 2-arm dose expansion portion of the Phase 1/2 clinical trial as proposed in this Phase IIb application. In the Phase 2 study, NMIBC patients will receive intravesicular instillation of either ALT-803 + BCG or BCG alone and the disease-free response rate, duration of recurrence, and localized and systemic immune activation will be assessed. Improved clinical outcomes using ALT-803 + BCG would support a Phase 3 evaluation of intravesical ALT-803 therapy in patients with NMIBC with the ultimate goal of developing more durable or curative therapies in this indication.